The Sundarbans Mangroves ecoregion is the world's largest mangrove ecosystem.

Named after the dominant mangrove species Heritiera fomes, locally known as sundri, this is the only mangrove ecoregion that harbors the Indo-Pacific region's largest predator, the tiger (Panthera tigris).

Unlike in other habitats, here tigers live and swim among the mangrove islands, where they hunt scarce prey such as chital deer (Cervus axis), barking deer (Muntiacus muntjak), wild pig (Sus scrofa), and even macaques (Macaca mulatta). Quite frequently, the people who venture into these impenetrable forests to gather honey, to fish, and to cut mangrove trees to make charcoal also fall victim to the tigers.

But the ecoregion's importance is not based solely on its role as a priority tiger conservation area. Mangroves are a transition from the marine to freshwater and terrestrial systems. They provide critical habitat for numerous species of fishes and crustaceans that are adapted to live, reproduce, and spend their juvenile lives among the tangled mass of roots, known as pneumatophores, that grow upward from the anaerobic mud to get the trees' supply of oxygen.

Location and General Description

Source: WWF

The ecoregion lies in the vast delta formed by the confluence of the Ganges, Brahmaputra, and Meghna rivers. The maze of mangrove channels extends across southern Bangladesh and India's West Bengal State. The June to September monsoon brings heavy rains and frequent, devastating cyclones that cause widespread destruction. Annual rainfall can exceed 3,500 millimeters (mm), and daytime temperatures can exceed a stifling 48⁰C during these monsoon months.

Mangroves are not diverse compared with most other terrestrial ecosystems. The undisturbed forests have an unstratified, dense canopy and an undergrowth made up of seedlings and saplings of the canopy trees. In the Sundarbans, the mangrove forests are characterized by Heritiera fomes, a species valued for its timber. Other species that make up the forest assemblage include Avicennia spp., Xylocarpus mekongensis, X. granatum, Sonneratia apetala, Bruguiera gymnorrhiza, Cereops decandra, Aegiceras corniculatum, Rhizophora mucronata, and the palm Nypa fruticans.

Source: WWF

Biodiversity Features

This vast mangrove ecosystem along the marine, freshwater, and terrestrial interfaces provides critical ecosystem functions. The tangled mass of roots from mangrove trees provides safe havens for juvenile stages of a gamut of species, from fish fry to shrimp naupleii.

The ecoregion harbors several mammals, but the most charismatic undoubtedly is the majestic Bengal tiger that swims from island to mangrove island searching for and hunting scarce prey. Because this ecoregion represents the only example of tigers ecologically adapted to a life in the mangroves, it has been designated a Level I TCU. The tiger's reputation as a human-eater is greater here than anywhere else in its range; the people who venture into the Sundarbans must take great precautions to avoid being attacked. One fascinating deterrent is wearing a mask in the back of the head because tigers are believed to be less liable to attack a human looking directly at them.

Several other predators dwell in this labyrinth of channels. Two species of crocodiles (Crocodylus porosus and C. palustris), the Gangetic gavial (Gavialis gangeticus), and the water monitor lizard (Varanus salvator) use both land and water to hunt and bask in. Sharks and the Gangetic freshwater dolphins (Platanista gangetica) inhabit the waterways. And several birds of prey patrol the sky overhead. More cryptic but equally fascinating are the mudskippers, a gobioid fish that climbs out of the water into mudflats and even climbs trees. An abundance of crabs, hermit crabs, and shrimp scavenge among the roots.

More than 170 bird species are known to inhabit these mangrove forests, including a single endemic species (table 1). This brown-winged kingfisher is limited to the coastal habitats in this ecoregion.

The bird assemblage also includes the globally threatened lesser adjutant (Leptoptilos javanicus) and threatened masked finfoot (Heliopais personata). There are twelve birds of prey that coexist here, including the osprey (Pandion haliaetus), white-bellied sea eagle (Haliaeetus leucogaster), and grey-headed fish-eagle (Ichthyophaga ichthyaetus). The mangrove ecosystem also is an important staging and wintering area for migratory birds, which include several species of shorebirds, gulls, and terns.

Current Status

Bangladesh supports one of the world's highest human population densities. About half of this ecoregion's mangrove forests have been cut down to supply the fuelwood and other natural resources extracted from these forests by this large population. Despite the intense and large-scale exploitation, the ecoregion still is one of the largest contiguous areas of mangroves in the world.

There are seven protected areas that cover almost 2,700 square kilometers (km²⁾, or 15 percent of the ecoregion (table 2). Despite the high proportion of the ecoregion being within the protected area system, only one of these, Sajnakhali, is large enough to support a space-dependent species such as the tiger. Many of the protected areas also lack trained and dedicated personnel and infrastructure to adequately manage them.

Types and Severity of Threats

The conservation status of the ecoregion was changed from vulnerable to endangered because of the projected human threats. The human population in the Sundarbans, now estimated at more than 2 million, continues to increase very rapidly. Hunting and trapping wildlife, cutting and lopping trees for fuelwood and to make charcoal, and overexploiting the trees for timber by the forestry industry are some of the most severe current threats. Shrimp fry are being collected at unsustainable levels to supply the shrimp grow-out industry, and the mangrove forests are cut and cleared to build shrimp grow-out ponds, contributing to degradation and habitat loss. There is also the potential for harmful effluents to enter the mangrove waterways from a proposed fertilizer plant.

But some of the greatest threats to this ecoregion's biodiversity emanate from thousands of kilometers away. The rivers that feed and flush the mangroves bring down heavy silt loads as a result of deforestation and erosion in the Himalayan Range. This silt and water turbidity have profound effects on the sensitive mangrove ecosystem and its flora and fauna, especially on the juvenile stages that the mangroves support. The diversion of more than 30 percent of the Ganges River's dry season flow through the Farraka Barrage in India to provide irrigation for agriculture has drastically increased salinity levels and disrupted fish migration and breeding patterns. The delicately balanced community composition in these mangroves is determined to a large part by salinity levels.

Additional Information on this Ecoregion

• For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
• To see the species that live in this ecoregion, including images and threat levels, see theWWF Wildfinder description of this ecoregion.
• World Wildlife Fund Homepage